A liquid crystal display (“LCD”) is a thin, flat display device made up of any number of color or monochrome pixels arrayed in front of a light source or reflector. It has many advantages over competing technologies because it uses very small amounts of electric power and is therefore suitable for use in battery-powered electronic devices, and because of its thinness.
Each pixel in an LCD consists of a layer of liquid crystal (“LC”) molecules suspended between two transparent electrodes, and sandwiched between two crossed linear polarizers (i.e., polarizers with axes of transmission which are perpendicular to each other). Without the liquid crystals between them, light passing through one polarizer would be blocked by the other. The liquid crystals act as polarization modifying light valves by changing the polarization state of the light coming from the rear polarizer. In order to function in this manner, the liquid crystal molecules must be correctly aligned so that they accept light of the polarization state transmitted by the rear polarizer and can rotate it to the polarization state that is transmitted by the front polarizer. Various techniques are known for achieving the appropriate alignment of the liquid crystal molecules. These include mechanical rubbing, which introduces microscopic grooves, or use of oriented linearly polarized UV illumination of an appropriate alignment layer substrate. Application of an electric field, by applying a voltage to the transparent electrodes, can modify the degree of polarization rotation, thus enabling fine control of the light passing through the pixel. The operation of LCDs depends on the correct relationship between the axis of transmission of the rear polarizer, the alignment of the liquid crystal layer (both for light entering and leaving) and the axis of transmission of the front polarizer.
The pixels by themselves do not generate light and therefore an LCD requires external illumination, either from ambient sources for a “reflective” LCD or from a backlight for a “transmissive” LCD (or from a combination in the case of a “transflective” LCD). A large portion of the power consumption of a transmissive LCD is devoted to the backlight. However one problem with known transmissive LCDs is that the vast majority of this power is expended in producing light that is ultimately not used in the display output, since it is filtered out. A typical light yield (i.e., the fraction of generated light that is transmitted by a fully active pixel) of these known LCDs is approximately 5%-7%.
Light loss that is intrinsic to LCD designs is typically due to the following elements (assuming illumination from an unpolarized white source):                color filter set: approximately 28% transmission;        aperture ratio: approximately 70% transmission; and        rear and front polarizers: approximately 40% transmission.Color filters are required since backlight typically generate white light. The aperture ratio arises since some of the area of an LCD does not transmit light.        
Polarization losses arise from intrinsic aspects of the design of LCDs. As has been described, LCDs require illumination to be linearly polarized and appropriately oriented, which typically results in the loss of at least half of the light available from the backlight.
Various attempts have been made to improve the light yield of LCDs, which could greatly improve the electrical efficiency of LCDs, therefore enabling more power efficient appliances, extending battery life for mobile devices, reducing needed backlight illumination components since fewer or lower power lamp elements would be needed to provide a certain level of brightness, and improving heat management in display units since much of the lost light is absorbed as heat. One method of improving light yield is through polarization light recycling, disclosed in, for example, U.S. Pat. No. 7,038,745. FIG. 1 is a cross-sectional view of a typical prior art transmissive LCD 10 which includes polarization light recycling in order to improve the light yield. FIG. 1 shows only those elements directly involved in light recycling and does not show many other elements often found in LCDs (e.g., color filters, prism sheet, diffusers, etc). LCD 10 includes a liquid crystal layer 17. Liquid crystal layer 17 is sandwiched by a homogenous front polarizer 19 and a homogenous rear reflective polarizer 18. Rear polarizer 18 transmits light of a first polarization (“P1”) and reflects light of a second polarization (“P2”). Front polarizer 19 is crossed with rear polarizer 18 (i.e., has perpendicular transmission axes) and transmits P2 light. Liquid crystal layer 17 functions as a plurality of polarization modifying light valves (i.e., pixels) each of which can rotate incident P1 light to include a P2 component, depending on the amount of applied voltage. LCD 10 further includes a backlight unit 11 that includes a backlight 12 for generating unpolarized white light, a rear reflector 14 for reflecting light, and a light guide 30 for guiding and homogenizing generated and reflected light.
In operation, backlight unit 11 produces a uniform distribution of white unpolarized illumination that includes both P1 and P2 light (arrow 20). This light is incident on homogenous rear reflecting polarizer 18 that transmits one polarization P1 (arrow 21) and reflects the majority of the other component, P2 (arrow 23), back into backlight unit 11. LC layer 17 converts some of the polarization of arrow 21 to P2 light and transmits that as arrow 24, of which the P2 light portion is ultimately transmitted by homogeneous front polarizer 19 as arrow 25.
Meanwhile, in backlight unit 11, elements such as rear reflector 14 convert P2 of arrow 23 so that it now has components of both P1 and P2 (arrow 26), thus allowing an increase in the amount of “recycled” P1 (arrow 27) available for transmission. A portion of the recycled P1 (arrow 27) is converted to recycled P2 (arrow 28) by LC layer 17, which is ultimately transmitted (arrow 29) by homogenous front polarizer 19. This process is repeated many times to increase the total amount of P2 light transmitted to the viewer. Commercial versions of the recycling technology shown in FIG. 1 are known to increase the light yield by approximately 30%.
Based on the foregoing, there is a need for an LCD system that has an improved light yield relative to known systems.